This was reported in IRC and I don't have a test case, but creating one should
be simple:
00:33 <bringert> darcs 2.0.2 overflows the stack when running 'darcs what' in a
repo where a small file has been replaced by a 3M text file
containing no newlines
00:34 <twb> bringert: did you build with GHC 6.8.3 or .2?
00:34 <bringert> 6.8.2
00:34 <twb> There are known issues with 6.8.2
00:36 ~~~ bringert upgrades ghc
<snip>
02:47 <bringert> darcs 2.0.2 compiled with GHC 6.8.3 has the same stack
overflow problem
My hunch is that is most likely lies in smart_diff or maybe some code that
creates a patch to display the results of whatsnew.

This is now fixed on the sprint branch (http://code.haskell.org/darcs/sprint/)
by either of the following two patches. They both make sense anyway, so I will
submit them both.
Sat Oct 25 13:23:31 BST 2008 Ganesh Sittampalam <ganesh@earth.li>
* just hash the last 20 characters in LCS
Sat Oct 25 13:18:57 BST 2008 Ganesh Sittampalam <ganesh@earth.li>
* accumulate hash strictly in hash/hashPS